Different Backpack on Mephiston?

Hey everyone, I picked up a Mephiston (Blood Angels Librarian) the other day, and as I was putting him together, I noticed that the standard backpack that he has looks kind of stupid on him. So, I decided to either get a new backpack, or file off the nub on his back. I've been thinking of using Kharn's backpack, which can be seen at: